M. E. Brandan

The interaction between light heavy-ions and what it tells us

Abstract Significant progress has been achieved during the last decade in our knowledge and understanding of the optical potential between two light heavy-ions. This has mostly been a consequence of the measurement of accurate and extensive elastic differential cross sections. Some of these data, covering over eight orders of magnitude in cross section, extend to sufficiently large scattering angles that they show remarkable refractive effects which remind one of features of the scattering of alpha particles by nuclei that have been known since the work of Goldberg some twenty years ago. Refractive effects, particularly nuclear rainbows, are evident in 12 C + 12 C and 16 O + 16 O angular distributions at bombarding energies between 6 and 100 MeV per nucleon. Their angular location and cross section have led to the determination of the gross features of the local optical potentials and in many cases have removed ambiguities in the depths of the real parts of the potentials. The resulting phenomenological potentials are strongly attractive (“deep”), with relatively weak absorption, and depend upon the bombarding energy. The optical model potential for such heavy-ions is no longer simply a way to parameterize scattering data (or perhaps just one of many ways). Ambiguities have been resolved, and a good understanding of the theoretical basis of its features has been attained. The folding model is central to this understanding, coupled with increased insight into the nature of realistic effective nucleon-nucleon interactions. This Report reviews the experimental evidence, its interpretation, and what we have learnt from it. Much of the interpretation becomes especially transparent when couched in the language of semiclassical scattering theory. We summarize this language, as well as the basic features of the theory of the optical model.

The use of a reflective scanner to study radiochromic film response.

GafChromic MD-55-2 film response was studied using a flatbed, reflective Microtek ScanMaker E3 scanner, under different conditions to optimize its use. Irradiations were performed using 60Co gamma rays in a 0-300 Gy dose interval to produce a calibrated step wedge. A 24-bit colour-scale mode along with image splitting into its red, green and blue components is suggested as an improved dosimetry method over a 256 grey level (8-bit) mode, extending the dynamic range for this film. Diverse film orientation and positioning, a black or a white background, and individual step film scanning were evaluated. Unwanted normalization is overcome by adding reference black and white steps adjacent to the radiochromic film, ensuring reproducibility. The use of a red filter was found to be equivalent to the use of the red component of the image after image colour splitting. The useful range for MD-55-2 film is extended up to 300 Gy if colour components, other than red, are used to evaluate the response. Comparisons with optical density measurements show that inexpensive commercial scanners might be a good alternative to densitometers.

Onset of supralinear response in TLD-100 exposed to 60Co gamma-rays

The onset of the supralinear response of peaks 4–9 in LiF:Mg, Ti (TLD-100) induced by 60Co gamma ray irradiation at doses from 4.18 mGy to 8.32 kGy has been studied reading the thermoluminescence signal at 8 °C s−1. Glow curves were decomposed into individual peaks by a controlled deconvolution procedure. The response of the low-temperature peaks (4 and 5) as a function of dose is linear up to ≈1000–5000 mGy and supralinear for higher doses; the high-temperature peaks (6a–9) exhibit a supralinear behaviour at doses higher than about 50–200 mGy. The f(D) supralinearity function has been evaluated, obtaining f(D)max equal to 3.6 ± 0.3 for peak 5 and from 23 up to 207 for the high temperature peaks. To assess the contribution to the glow curve from the high-temperature peaks, two methods were studied: ratios of peak heights (peak 7 with respect to peak 5) and ratios of areas of the deconvoluted high-temperature peaks (added area of peaks 6b, 7 and 8) with respect to peak 5. The shape of the glow curve ceases to be constant, abruptly, at a dose near 100 mGy, if the ratio of areas is evaluated.

Thermoluminescent response and relative efficiency of TLD-100 exposed to low-energy x-rays.

The dose-response of LiF:Mg,Ti (TLD-100) exposed to 15 and 35 kVp (8.0 +/- 0.1 and 8.1 +/- 0.1 keV effective energy respectively) x-rays and 60Co gamma-rays has been measured in the dose interval from (1.2-5.4) x 10(3) Gy for x-rays, and from 0.14 to 850 Gy for gamma-rays. In both cases the total TL signal and glow curve peaks 3 to 9 show supralinearity. The supralinearity function f(D) is similar for both x-ray beams, except for peak 8, where a 30% difference is observed. The maxima of f(D) for the total TL signal and peaks 5 to 8 are 2.1, 1.7, 6.4, 3.3 and 7.5 respectively for 8.1 keV x-rays and 3.7, 3.1, 13.6, 9.9 and 11.0 for gamma-rays. The measured relative efficiencies for x-rays with respect to 60Co, for the total TL signal and peaks 5 and 7, were 1.04, 0.97 and 3.2 respectively.

Analytical optimization of digital subtraction mammography with contrast medium using a commercial unit

Contrast-medium-enhanced digital mammography (CEDM) is an image subtraction technique which might help unmasking lesions embedded in very dense breasts. Previous works have stated the feasibility of CEDM and the imperative need of radiological optimization. This work presents an extension of a former analytical formalism to predict contrast-to-noise ratio (CNR) in subtracted mammograms. The goal is to optimize radiological parameters available in a clinical mammographic unit (x-ray tube anode/filter combination, voltage, and loading) by maximizing CNR and minimizing total mean glandular dose (D(gT)), simulating the experimental application of an iodine-based contrast medium and the image subtraction under dual-energy nontemporal, and single- or dual-energy temporal modalities. Total breast-entrance air kerma is limited to a fixed 8.76 mGy (1 R, similar to screening studies). Mathematical expressions obtained from the formalism are evaluated using computed mammographic x-ray spectra attenuated by an adipose/glandular breast containing an elongated structure filled with an iodinated solution in various concentrations. A systematic study of contrast, its associated variance, and CNR for different spectral combinations is performed, concluding in the proposal of optimum x-ray spectra. The linearity between contrast in subtracted images and iodine mass thickness is proven, including the determination of iodine visualization limits based on Roses detection criterion. Finally, total breast-entrance air kerma is distributed between both images in various proportions in order to maximize the figure of merit CNR2/D(gT). Predicted results indicate the advantage of temporal subtraction (either single- or dual-energy modalities) with optimum parameters corresponding to high-voltage, strongly hardened Rh/Rh spectra. For temporal techniques, CNR was found to depend mostly on the energy of the iodinated image, and thus reduction in D(gT) could be achieved if the spectral energy of the noniodinated image is decreased and the breast-entrance air kerma is evenly distributed between both acquisitions. Predicted limits, in terms of iodine concentration, are found to guarantee the visualization of common clinical angiogenic concentrations in the breast.

Use of an orthovoltage X-ray treatment unit as a radiation research system in a small-animal cancer model.

BackgroundWe explore the use of a clinical orthovoltage X-ray treatment unit as a small-animal radiation therapy system in a tumoral model of cervical cancer.MethodsNude mice were subcutaneously inoculated with 5 × 106 HeLa cells in both lower limbs. When tumor volume approximated 200 mm3 treatment was initiated. Animals received four 2 mg/kg intraperitoneal cycles (1/week) of cisplatin and/or 6.25 mg/kg of gemcitabine, concomitant with radiotherapy. Tumors were exposed to 2.5 Gy/day nominal surface doses (20 days) of 150 kV X-rays. Lead collimators with circular apertures (0.5 to 1.5 cm diameter) were manufactured and mounted on the applicator cone to restrict the X-ray beam onto tumors. X-ray penetration and conformality were evaluated by measuring dose at the surface and behind the tumor lobe by using HS GafChromic film. Relative changes in tumor volume (RTV) and a clonogenic assay were used to evaluate the therapeutic response of the tumor, and relative weight loss was used to assess toxicity of the treatments.ResultsNo measurable dose was delivered outside of the collimator apertures. The analysis suggests that dose inhomogeneities in the tumor reach up to ± 11.5% around the mean tumor dose value, which was estimated as 2.2 Gy/day. Evaluation of the RTV showed a significant reduction of the tumor volume as consequence of the chemoradiotherapy treatment; results also show that toxicity was well tolerated by the animals.ConclusionResults and procedures described in the present work have shown the usefulness and convenience of the orthovoltage X-ray system for animal model radiotherapy protocols.

Evaluation of dual-energy subtraction of digital mammography images under conditions found in a commercial unit

Radiological contrast-to-noise ratio (CNR) is evaluated in subtracted images of microcalcifications in breast tissue. CNR is calculated for dual-kVp subtraction combining beams available in a Senographe 2000D, assuming single breast compression. Spectra were obtained from Boone et al (1997 Med. Phys. 24 1863-73), and the study was limited to lowest 25 kV Mo/Mo and highest 40 kV Rh/Rh beams, for 2.58 x 10(-4) C kg(-1) (1R) total exposure. For a standard case combining 25 kVp Mo/Mo and 40 kVp Rh/Rh beams, predicted maximum CNR for 300 microm calcification in 5 cm thick, 50% glandular, breast is about 1.2, below Roses criterion for visualization. Total mean glandular doses are about 2.5 cGy for a standard case. The effect that input factors might have on predictions has been evaluated. Choice between alternative spectra can affect CNR by 50%. Assumed calcification composition leads to differences of 67% in calculated CNR, and assumed breast tissue composition can alter CNR by 45%; these results are weakly dependent on calcification or breast thickness, or on the assumed fraction of glandular tissue. CNR values are related to detected spectra effective energy. Calculations predict that above 37 kVp Mo/Mo beams are more energetic than Rh/Rh at the same kVp, due to beam hardening.

Measurement of the thermoluminescent response (supralinearity and efficiency) of LiF : Mg,Ti exposed to 0.7 MeV protons

Abstract The irradiation of LiF : Mg,Ti (TLD-100) chips by a defocused 0.7 MeV proton beam from an electrostatic accelerator has been achieved using radiochromic dye film to evaluate the beam transversal intensity distribution. The dose-response has been measured at fluences between 2.4 × 108 and 5.7 × 1011 p/cm2 and peaks 3–9 show linear-supralinear-sublinear response. The efficiency, relative to 60Co γ-rays, measured for the total thermoluminescent (TL) signal and peaks 5 and 7, equals 0.33, 0.22 and 1.4, respectively. The comparison of these and other low-energy data for peak 5 with Track Structure Theory (TST) efficiency calculations indicate that the latter must take into account the stopping of the incident ions in the dosimeter to achieve a reasonable agreement with the observations. In general, neither TST, nor Modified Track Structure Theory (MTST) are able to simultaneously predict the response to 5.3 MeV α-particles and 0.7 MeV protons for all the studied glow curve peaks. Track Interaction Model (TIM) and Unified Track/Defect Interaction Model. (UNIM) calculations of the supralinearity function f(D) of peaks 5 to 8 for 5.3 MeV α-particles and peaks 5 to 9 for 0.7 MeV protons, qualitatively describe the general trends measured in this work; only the UNIM agrees quantitatively with the data in both systems.

Observation of enhanced efficiency in the excitation of ion-induced LiF:Mg,Ti thermoluminescent peaks

LiF:Mg,Ti thermoluminescent relative efficiency (RE), with respect to Co60 gammas, induced by H1, He3, C12, O16, and Ne20 has been measured as a function of linear energy transfer (LET) from 1.55to517keV∕μm. RE for high-temperature peaks are >1 (up to 20), displaying maxima at 20–300keV∕μm. RE for peaks 4 and 5 decreases monotonically from 1, with increasing LET. A correlation is observed between supralinearity in the gamma response and enhanced efficiency to ions. Katz track structure theory [J. J. Butts and R. Katz, Radiat. Res. 30, 865 (1967)] reproduces the main features of the ion data, though it overestimates RE for the lighter ions. The agreement of the model fits and the measurements does not permit the evaluation of “sensitive site” sizes. On the other hand, Horowitz modified track structure theory [J. Kalef-Ezra and Y. S. Horowitz, Int. J. Appl. Radiat. Isot. 33, 1085 (1982)] describes the global trends in the data, particularly the high efficiencies observed for the high temperature peaks if a ...

La calidad de los servicios de radiología en cinco países latinoamericanos

OBJETIVO: Determinar la correlacion entre ciertos indicadores de calidad para los servicios de imaginologia y la certeza en la interpretacion de los examenes radiologicos para cuatro quejas frecuentes: las masas de la mama, el malestar del aparato digestivo, el dolor de espalda y los sintomas de la tuberculosis. METODOS: Se evaluaron veintiseis servicios de radiologia en Argentina, Bolivia, Colombia, Cuba y Mexico. Se evaluaron los equipos de mamografia y de radiografia/fluoroscopia convencional usados en los servicios seleccionados utilizando protocolos comunes, hojas de especificaciones tecnicas, instrumen-tos de prueba, maniquies y sistemas de dosimetria calibrados. Los estudios se realizaron en establecimientos de complejidad media. Se obtuvo el consentimiento informado de todos los pacientes estudiados, y se garantizo la confidencialidad de los resultados. Se evaluaron y documentaron los siguientes parametros: el tipo de establecimiento (publico o privado); la poblacion cubierta; el numero de pacientes y examenes; los equipos radiologicos, los de procesamiento de imagenes y los suministros; la educacion y la capacitacion del personal profesional y tecnico; los programas de la garantia de la calidad y del mantenimiento preventivo, y la adherencia a las normas de seguridad radiologica. Se determinaron el funcionamiento de los equipos de rayos X, los receptores de la imagen y las procesadoras; las condiciones del cuarto oscuro y de la visualizacion de las imagenes; las dosis recibidas por los pacientes y la calidad de la imagen, usando parametros uniformados en todos los casos. Los paneles independientes de radiologos, reconocidos como expertos por la sociedad radiologica local, evaluaron la calidad de las imagenes clinicas obtenidas y realizaron una interpretacion radiologica para cada paciente usando las mismas peliculas e historia clinica a disposicion de los medicos especialistas en imaginologia de la institucion. El acuerdo entre los informes de los paneles de expertos y los de los radiologos locales se tomo como un indicador de la certeza de la interpretacion radiologica. RESULTADOS: Se analizaron 366 mamografias, 343 procedimientos radiologicos para las quejas del aparato digestivo, 319 examenes de rayos X de la columna vertebral y 157 radiografias de torax. El acuerdo entre la interpretacion radiologica del panel de expertos y del medico local fue de 70% a 100%, excepto en el caso de las peliculas de la columna vertebral en Cuba (57,8%) y de las mamografias en Mexico (33,3%), que el panel de expertos juzgo estaban entre las imagenes clinicas de peor calidad. Se encontro una correlacion positiva significativa entre la certeza en la interpretacion radiologica y la calidad de las imagenes radiologicas. La calidad de la imagen mostro una correlacion positiva con el nivel de formacion y capacitacion de los tecnicos. Los estudios que se realizaron en los servicios que contaban con equipos automaticos de revelado y que cumplieron con los indicadores establecidos para el contacto pelicula-pantalla obtuvieron imagenes de mejor calidad y una proporcion mayor de estudios con resultados concordantes. Mas de 50% de los negatoscopios no cumplieron con los criterios de calidad para el brillo y la homogeneidad. CONCLUSIONES: Una imagen de buena calidad es fundamental para el logro de un diagnostico certero. Se debe hacer hincapie en la educacion continuada de los tecnicos de radiologia y en la adquisicion y el mantenimiento de los equipos y los accesorios adecuados, especialmente de los negatoscopios, las pantallas intensificadoras y las maquinas reveladoras automaticas, dada la repercusion que tienen en la calidad de las imagenes.